Secondary batteries are used in electric vehicles and hybrid electric vehicles, for example, to supply their electric motors for driving the vehicles with electric power (electric discharge) and to store the electric energy obtained from the electric motors functioning as a generator that changes brake energy into the electric power, or to charge the electric energy that is supplied from power source equipment on the ground.
In this case, in order to maintain the states of the secondary batteries to be optimal over the long term, it is needed to always monitor the states of the batteries, especially the states of charge (SoC) thereof and thereby carry out battery management.
The states of the batteries, however, cannot be directly estimated when the batteries are secondary batteries because the discharge and the charge occur under chemical action. In this case, the temperature change affects strongly on the states of the batteries, which always change in their usage environment and according to the usage history.On the other hand, the capacitors are known as another means to obtain the action similar to that of the batteries. In the capacitors, the states thereof can be almost certainly detected by the measurement of the discharge and charge current, the terminal voltage and so on, because the discharge and charge of the capacitors occur under physical action. The capacities of the capacitors are, however, smaller than those of the batteries, and accordingly the secondary batteries used overwhelmingly as a main power source of the vehicles except a part of trolleybuses and the likes.Therefore, various methods for estimating the states of charge of the secondary batteries have been proposed.
As the conventional means for estimating the state of charge of the secondary battery, a bookkeeping method (namely a current integration method, or a Coulomb counting method) is known. In this method, the output and input voltage and current of the battery are stored in the form of time-series data, and the current is time-integrated to obtain present the electric charge based on the data, and then the SoC is computed by using the initial charge of the battery and its full charge capacity.
The method, however, needs to always monitor the state of the battery, and it is difficult to become normal again once the estimation of the SoC deviates because the errors keep on being accumulated. In addition, it is necessary to gather a lot of experiment data in advance.Therefore, the other method is proposed.
Another method is known, where the discharge and charge current and the terminal voltage of the secondary battery is measured, and all of the coefficient parameters are estimated together from the current and the terminal current based on a battery model of the secondary battery by using a parameter estimating device containing an adaptive digital filter. Then the open-circuit voltage is computed from the estimated results, and the state of the battery is estimated based on a relationship, which is obtained in advance, between the open-circuit voltage and the stage of battery.